1. Field of Invention
This invention relates to oil filters and more particularly to such filters which add viscosity index improver to oil over its normal useful life.
2. Prior Art
It is well known that mineral lubricating oils and functional fluids have a tendency to become thin at elevated temperatures while becoming thick at low temperatures, and thus it is generally necessary to incorporate additives which improve their viscosity-temperature relationships. For example, in the case of a crankcase lubricating oil in a cold engine, it is desirable that the oil not become so thick that it is difficult to start the engine. On the other hand, when the engine is hot, it is necessary that the oil be sufficiently viscous that an oil film is maintained between the moving parts.
The viscosity-temperature relationship of an oil at temperatures in the range of 110.degree. to 210.degree.F. is known as its viscosity index. Thus, additives which retard the tendency of the oil to thin as the temperature is raised from 100.degree. to 210.degree.F. are known as viscosity index improvers. Viscosity index improvers which are known include polymers of methacrylate esters having long alkyl chains, polyisobutylene polymers, and polyalkyl styrenes. The methacrylate type improvers are described in U.S. Pat. No. 2,114,233, issued Apr. 12, 1938 to Harry Trainor et al. and U.S. Pat. No. 2,737,496, issued Mar. 6, 1956 to Willard E. Catlin. The polyisobutylene improver is described in U.S. Pat. No. 2,130,507, issued Sept. 20, 1938 to Michael Otto et al. Polyalkyl stryene improvers are described in U.S. Pat. No. 2,072,120, issued Mar. 2, 1937 to Louis A. Mikeska et al. Many patents have issued subsequently to those above described which disclose many types of additives which can be included with the improver.
Viscosity index improvers which are widely used at the present time are crude ethylene-propylene copolymers which are dissolved in mineral oil or lubricating oil to form a concentrate and the resulting viscous solution added to motor oils and other oils as viscosity index improvers. Such material are described in U.S. Pat. No. 3,598,738, issued to Charles B. Biswell at al. on Aug. 10, 1971; U.S. Pat. No. 3,560,384, issued Feb. 2, 1971, to Robert A. Halling and U.S. Pat. No. 3,679,830, issued July 25, 1972, to Charles B. Biswell et al.
In present practice where the viscosity index improver is added to lubricating oil, the contribution of the improver immediately begins to deteriorate during operation. Thus, at some point prior to the end of the useful life of the oil, the oil has thinned out and the improver is no longer effective. U.S. Pat. No. 3,336,223, issued Aug. 15, 1967, to P. D. Kneeland teaches an oil filter useful for maintaining an effective concentration of additives in oil.